Step 1

If you're using dried pasta, bring a large pot of salted water to a boil. If you're using fresh pasta sheets, cut them into long wide noodles approximately 3 inches by 13 inches, or a size that will fit into your lasagna dish. Cook the noodles (6 at a time for dried noodles) until they are tender but still underdone (they will finish cooking as the lasagna bakes); fresh pasta will take only a minute. Drain and then lay the noodles flat on a towel so they won't stick.

Step 2

Heat the oven to 400 degrees. Grease a rectangular baking dish with the olive oil, add a large dollop of tomato sauce and spread it around. Put a layer of noodles (use four) in the dish, trimming any overhanging edges; top with a layer of tomato sauce, one-third of the spinach, and one-fourth of the ricotta (use your fingers to spread it evenly), the mozzarella and the Parmesan. Season with salt and pepper if desired.

Step 3

Repeat the layers twice, and top with the remaining noodles, tomato sauce, ricotta, mozzarella and Parmesan; the top should be covered with cheese; add more ricotta and Parmesan as needed. (The lasagna may be made ahead to this point, wrapped tightly and refrigerated for up to a day or frozen. Bring to room temperature before proceeding.)

Step 4

Bake until the lasagna is bubbling and the cheese is melted and lightly browned on top, about 30 minutes. Remove from the oven and let rest a few minutes before serving, or cool completely, cover well, and refrigerate for up to 3 days, or freeze.